Page 98 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

84 Principles and Methods

in C 60 is sp 2.278 and a fractional s character of 0.085, which results in the
orbitals extending further beyond the outer surface than into the inte-
rior of fullerene. This analysis implies that fullerenes, especially C 60 , are
fairly electronegative molecules as well as having the low lying *
orbital with considerable s character. Indeed, the reactivity of fullerene
can be considered as a fairly localized, but electron deficient, polyolefin
void of substitution reaction. This trend in terms of electrophilicity
allows for the ready chemical reduction and nucleophilic addition to
fullerenes.
Theoretical calculations show that the LUMO (t 1u symmetry) and
LUMO 1 (t 1g symmetry) of C 60 molecular orbitals exhibit a relatively
low lying energy and are triply degenerated. So C 60 was predicted to
be a fairly electronegative molecule that can be reduced up to hexan-
ion. Cyclic voltammetry (CV) studies show that C 60 can be reduced and
oxidized reversibly up to 6 electrons with one-electron transfer
processes. Indeed, reduction reactions were the first chemical trans-
formation carried out to C 60 . Fulleride anions can be generated by
electrochemical method and then be used to synthesize covalent
organofullerene derivatives by quenching the anions with elec-
trophiles. Alkali metals can chemically reduce fullerene in solution and
solid state. It is with the alkali metal doped K 3 C 6 that superconduc-
tivity was first found in fullerene materials. Alkaline earth metals
can also be intercalated with C 60 by direct reaction of C 60 with alka-
line earth metal vapor to form M x C 60 (x 3 6), which possess super-
conductivity as well. Besides, C60 can also be reduced by less
2
ectropositive metals such as mercury to form C 60 and C 60 . In addition,
fulleride salts can also be synthesized with organic molecules to form
fullerene based charge transfer (CT) complexes. The well-known exam-

ple of this type is [TDAE ][C 60 ], which possesses remarkable elec-
tronic and magnetic behavior.
As stated above, geometric and electronic analysis predicted that
fullerene behaves like an electro-poor conjugated polyolefin. Indeed, C 60
and C 70 undergo various nucleophilic reactions with carbon, nitrogen,
phosphorous, and oxygen nucleophiles. C 60 reacts readily with organo-
lithium and Grignard compounds to form alkyl, phenyl, or alkanyl
fullerenes. However, one of the most widely used nucleophilic additions
to fullerene is the Bingel reaction (Figure 3.39), where a carbon nucle-
ophile was generated by deprotonation of -halo malonate esters or
ketones and added to form a clean cyclopropanation of C 60 with a 30 ~ 60
percent yield. Later, it was found that the -halo esters and ketones can
be generated in situ with I 2 or CBr 4 and a weak base as 1,8-
diazabicyclo[5.4.0]undec-7ene (DBU). This further simplified the reac-
tion procedures. The Bingel reaction is considered one of the most versatile